Power-based Comminution Calculations Using Ausgrind

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Power-based comminution calculations using Ausgrind

Greg Lane, Bianca Foggiatto and Marcos Bueno

Ausenco Services Ltd., Australia

ABSTRACT

Ausgrind is Ausenco’s in-house program used for power-based comminution calculations. Proprietary

calculations have been developed and reconciled with unit process operation and benchmarked with

plant performance.Ore breakage data from characterization and variability testwork, equipment geometry, flowsheet

configuration, efficiency factors and operating conditions are inputs. Ausgrind contains a database of

operating data and circuit types from many operations and over a wide range of ore types and

throughputs. These are referenced when developing flowsheets and circuit efficiency factors which

correct the specific energy required to grind from the nominated feed size to the specified product size. As

circuit efficiency factors are determined in Ausgrind, correction factors such as those listed by Bond and

Rowland are not employed.

The principal outputs are the total circuit specific energy and the specific energy for each of the

components in the comminution circuit. Ausgrind provides a valuable tool for evaluating comminution

options and optimizing circuit and equipment configurations.

Comminution is a pivotal and critical component in most mineral processing operations for several

reasons: it liberates the valuable mineral rendering it amenable to subsequent processes; it represents one

of the highest capital cost investments for unit processes in the treatment plant; and it contributes a

significant component to operating cost due to its high energy requirements. Therefore, determining the

most cost-effective circuit and predicting the energy requirement for each unit process to achieve the

desired size reduction in the product stream from the comminution circuit are critical tasks for circuit

design.

This paper describes the Ausgrind approach for estimating power and design of comminution circuits

and compares its outcomes to other published approaches. Practical examples are also presented, showing

applications for different circuit configurations for comminution operations, such as multi-stage crushing,

SAG and ball mill-based circuits.

INTRODUCTION

The progressive decrease in ore grades in the past decades has led the minerals industry to process ore at

higher rates to achieve the same metal production, driving significant increases in energy consumption.

Determining the most cost-effective circuit and predicting the energy requirement for comminution

circuits has become an even more critical task in concentrator design.



Von Rittinger (1867 apud Bond, 1961) and Kick (1885 apud Bond, 1961) proposed theoretical comminution

equations based on surface area and particle volume respectively, while Bond (1952) suggested an

empirical relationship that related the power requirement to the length of new cracks formed. Bond’s

equation was developed for the purpose of designing conventional crushing and grinding circuits (crush-

rod-ball mill or crush-ball mill). By the 1970s, the application of conventional circuits tended to be limited

to relatively low capacities. Since then, circuits featuring autogenous (AG) and semi-autogenous (SAG)

mills, often combined with ball mills, have developed a prominent status in the minerals industry. Several

methodologies have been established for designing AG/SAG based grinding circuits and predicting their

energy requirements.

The most frequently used methodologies for calculating energy requirements of comminution circuits

including AG/SAG mills are briefly described in the following section. This paper focuses on the

description of the Ausgrind approach and also compares its outcomes to other published approaches.

Practical examples are presented, showing measured power draw and Ausgrind predictions for different

SAG and ball mill-based circuit configurations.

EXISTING POWER-BASED CALCULATIONS FOR COMMINUTION CIRCUITS

Most modern grinding circuits consist of a primary SAG mill and secondary ball mill. Bond’s method is

widely used for determining the specific energy (Ecs) for crushers, rod mills and ball mills, whilst for

AG/SAG mills, several methodologies have been proposed over recent decades. Bond’s approach consists

of determining the ore characteristics via standardized laboratory testwork, then calculating the Ecs and

the power requirement based on throughput, and finally sizing the industrial machine. Similar to Bond’s

approach, most methodologies for sizing AG/SAG mills involve bench scale testwork for determining Ecs

and empirical equations for calculating mill power and throughput. Some of those methodologies are

briefly described as follows:

MacPherson (1978) created a grinding test that was conducted in a continuous laboratory mill in

closed circuit with a classifier until steady state was achieved. At test completion, size analyses were

carried out and the Ecs computed using power draw and throughput information. The test also

provided insight into how the harder components built up in the mill charge.

Barratt and Allan (1986) proposed the utilization of Bond comminution work indices and an efficiency

factor (fSAG) for determining the AG/SAG circuit Ecs. Using a pilot plant and operating plant database,

Barratt (1989) developed an empirical formula to determine the power requirements for circuits

including SAG and ball mills. The method required a proposed F 80 feed size to the SAG mill, the final

grind size and an estimate of the transfer size between the two mills. This methodology wasincorporated into a computer-based program for the sizing and selection of grinding circuits called

GrindPower (Matthews and Barratt, 1991).

The JKMRC methodology is based on the use of the JKSimMet program (Napier-Munn et al., 2005).

The program includes comminution “black box” models that are calibrated with equipment and

survey data, as well as ore specific parameters such as the drop weight test (JKDWT) “A×b” and the

Bond ball mill work index. The SAG and ball mills power draw is calculated using semi-empirical



equations, as a function of mill geometry, speed and load. The SAG Power Index (SPI) test was developed by Starkey and Dobby (Starkey and Dobby, 1996) as

the AG/SAG equivalent to the work index test. A SAG mill is operated in closed circuit and the time

required to grind the material to a P 80 of 1.7 mm is used to predict AG/SAG Ecs using power-based

models available within CEET (Comminution Economic Evaluation Tool).

The SAGDesign test was developed by Starkey, Dawson Lab and Outotec to overcome technical

limitations of the SPI (Starkey et al, 2006). The SAGDesign test procedure adopted a laboratory SAG

mill operated in closed circuit. The ground material generated by the SAG mill is used in a subsequent

ball mill Bond-style work index test. By conducting these two tests, the total grinding energy can be

calculated.

Morrell (2004) developed a methodology for predicting grinding circuit Ecs that used two workindices: one related to a coarse range (stages prior to ball milling) and the other to a fine range

(conventional ball milling). The size selected as the threshold between coarse and fine grinding was

0.75 mm. SMC test results are used to determine drop-weight indices (DWi). The work index related

to coarse grinding (Mia) is calculated from the data output of the SMC test. The Bond ball mill work

index test is used to calculate the work index related to fine grinding (Mib). Mia and Mib are used to

calculate Ecs for the coarse (Wa) and fine (Wb) components according to the equations published by

Morrell (2009). The sum of Wa and Wb results in the total Ecs to reduce the crusher product to the ball

mill product.

Sidall and Putland (2007) published the OMC (Orway Mineral Consultants) method to calculate

grinding power requirement, based on laboratory tests that defined the high and low energy breakage

characteristics. High energy breakage tests such as the autogenous media competency test (AMCT), JKDWT, SPI or SAGDesign test are twinned with a low energy breakage test such as Bond ball work

index. Standardize feed and product sizes are used to calculate the overall energy requirements using

Bond’s formula and a fSAG efficiency factor. The OMC database is then used to select viable options of

circuit configuration for detailed analysis.

Burgess (2012) developed the DBC (Don Burgess Consulting) methodology for calculating grinding

energy using Bond’s comminution theory, which is used to calculate the Ecs of individual

comminution ranges using breakage characterization indexes obtained from Bond’s crusher, rod and

ball mill tests and the JKDWT. The parameters obtained from the JKDWT are converted to

grindability work indexes and AG/SAG Ecs is calculated and corrected when variations occur to

standard parameters such as specific gravity, A×b values and mill aspect ratio.

AG/SAG CIRCUIT SPECIFIC ENERGY (ECS) AND EFFICIENCY FACTORS

The methods available for determining the Ecs requirements for AG/SAG based circuits rely on

combinations of ore breakage tests and Bond-style power based models. The JKMRC methodology is the

only commonly used method that uses a breakage/classification model with empirical factors. The use of

Bond work indices associated with other breakage tests has become a standard for measuring ore



hardness and establishing comminution energy requirements. Nevertheless, all these methods rely on theuse of efficiency factors to correct the comminution circuit energy requirements. Ausgrind also uses the

Bond work indices, along with JKDWT, SMC, SPI or MacPherson index, as inputs for calculating the

comminution circuit Ecs.

The empirical equations for determining the efficiency factor (fSAG) in Ausgrind calculations were derived

from Ausenco’s database (Bueno and Lane, 2011). The fSAG relationships are ore-dependent and specific to

the selected circuit configuration. They may also be adjusted when secondary crushing or intensive

blasting practices are in place (i.e. for finer circuit feed sizes, F80). Figure 1 shows the Ausgrind fSAG

relationships for different circuit configurations and feed sizes. Likewise, an f SAG relationship was

calculated for Morrell’s method (2009) as the ratio between the total Ecs using SMC results and Bond’s

formula; and is plotted in Figure 1 for comparison. In contrast to Ausgrind, the published methodology

used by Morrell (2009) assumed the same circuit efficiency (fSAG) for different circuit configurations, but

also varied with ore hardness (DWi).

Figure 1 Ausgrind fSAG and efficiency improvement due to size reduction

THE AUSGRIND APPROACH

Ausgrind is a method used by Ausenco for designing comminution circuits. It contains published and in-

house empirical formulae for calculating the specific energy of the equipment featured in comminution

circuits. The Ausenco approach is summarized in a block diagram shown in Figure 2, and described in the

following sections.



Figure 2 Ausenco methodology for comminution circuit design

Project Concept Definition

Determining the project concept is a fundamental activity that establishes the operating basis and

precedes all other tasks when designing a comminution circuit. Initial considerations to define the project

concept must contemplate the location of the orebody, nature of the ore (preliminary characteristics), local

environmental restrictions, product specifications as well as water availability and energy costs. Based on

such considerations, a conceptual processing route to obtain the plant final product is established. Table 1

shows a matrix for preliminary circuit selection which is reflects plant size (throughput) and ore

competency relationships.

Table 1 Matrix for circuit selection based on throughput and competency (Updated from Lane et al.,2002)

Throughput High Competency Moderate Competency Low Competency

<0.5 Mt/a Stage crush with ball millOne or two stage crush to SAG or

ball mill

Single stage SAG

mill

0.5 to 2 Mt/aStage crush with ball mill or

SAG and ball mill with pebble crushSAG and ball mill

Single stage SAG

mill or

SAG and ball mill

1 to 5 Mt/a SAG and ball mill with pebble crushSAG and ball mill or

AG and ball mill with pebble crush

Single stage SAG

mill or

SAG and ball mill

5 to 10 Mt/a SAG and ball mill with pebble crushSAG and ball mill or

AG and ball mill with pebble crushSAG and ball mill

>10 Mt/a SAG and ball mill with pebble crush orStage crush, HPGR and ball mill SAG and ball mill with pebblecrush SAG and ball mill

This matrix is used as an indication for circuit selection. However, it must be carefully adapted when

problematic ore types present in the deposit can influence the performance of specific stages of the plant.

Sample Selection

Once the preliminary project concepts are defined, the next step is to establish the attributes of the



testwork program. The testwork program must include appropriate ore characterization tests that arechosen to describe the orebody breakage properties. A detailed evaluation of geology through the

inspection of drill cores and geological sections is required when defining the comminution testwork

program. These inspections allow identification of relationships between the variability of ore

competency/hardness and the lithologies, rock types and mineralization.

The number and type of samples is dependent on the size and status of the project, project risk profile,

acceptable contingencies in defining equipment sizes, capital cost and operating cost. Typically, the

testwork program is divided into three phases. The first phase occurs during the scoping and

prefeasibility studies and samples are selected on the basis of major known rock types. The second and

third phases occur during the feasibility study. The second phase focusses on variability analysis across

the orebody where discrete samples are tested and compared with geometallurgical indicators.

Throughput and operating costs are forecasted more accurately with a relatively higher amount of

samples. For the third phase, samples are composited on the basis of spatial representativity, mine period

production, or hard wired ore typing. Table 2 shows the recommended number of samples for the last

phase of the testwork.

Table 2 Required number of samples during feasibility studies

Basis Number of samples Comments

spatial representativity >100 includes samples from Phase 1 and 2

mine period production 5 to 8 excludes samples from Phase 1 and 2

hard wired ore typing 10 to 30 includes samples from Phase 1 and 2

Breakage Characterization Testwork

As described in the previous sections, several methods are available for testing breakage characteristics.

Ausgrind typically uses results from the following tests, depending on the circuit configuration under

evaluation: Bond tests (crushing - CWI, rod mill - RWI, ball mill - BWI), SMC test and/or JKDWT.

The SMC test and the JKDWT directly yield values of DWi and A×b. The DWi is obtained from the

relationship between input energy (kWh/t), specific gravity and a size distribution index (t 10) obtained for

the SMC test or JKDWT product. The DWi is directly related to the JKMRC rock breakage parameters A

and b. As the SMC test does not use particles greater than 31.5 mm, its results can be biased when testing

extremely competent ores (A×b values lower than 35). Therefore, JKDWT data are typically used to

calibrate the SMC results. If breakage characterization testwork was conducted using other methodologies

such as the MacPherson’s autogenous milling test or SPI, empirical formulae based on Ausenco’s database

are used to correlate the outcomes with Ausgrind model input parameters.

Comminution Circuit Energy

The Ausgrind program calculates the total Ecs for the grinding circuit as a product of the calculated Bond

Ecs and the energy efficiency factor (fSAG). The Bond Ecs is calculated using the CWi, RWi, and BWi to

determine the specify energy requirements of a crush-rod-ball mill circuit for a P 80 of 150 microns as per

the Bond formulae without any correction factors such as those listed by Bond, Rowland and others. If the



product size (P80

) is different to 150 microns, the energy difference is calculated as per the Bondrelationship. The Ausgrind total Ecs calculation is described in the equation below.

Total Ecs = [(Bond Ecs to 150µm) * (f SAG – F80_effect)] ± [Bond Ecs to final P80]

The base case SAG mill Ecs is calculated as function of ore competency (in this case as measured by the

DWi), as shown in Figure 4.

Figure 4 SAG Ecs relationship with ore hardness (DWi)

A series of factors are then applied to adjust the base case SAG Ecs for the operating conditions, mill

dimensions (e.g. aspect ratio) and others. Some of these adjusting factors are shown in Figure 5 and the

SAG Ecs calculation is described in the equation below.

SAG Ecs = [Base Case SAG Ecs] * [adjusting factors]

Figure 5 Adjusting factors to SAG Ecs

The pebble crusher Ecs is calculated separately and vendor data is used for equipment selection.

The ball mill specific energy is calculated as the residual using the following equation:



Ball mill Ecs = Total grinding circuit Ecs – SAG mill EcsFor circuits without a SAG mill, the ball mill specific energy equals to the total grinding circuit specific

energy. Similarly, for circuits without ball mills, the SAG mill specific energy equals to the total grinding

circuit specific energy. Mill shell and motor size selection considers the specific energy requirements and

the specified plant throughput, together with the expected operating envelope of each mill.

BENCHMARKING AUSGRIND

As several tests have been developed to suit the requirements of AG/SAG circuit design, the different

testwork methods and interpretation of their results can produce very different outcomes. Differences in

SAG mill Ecs determination and throughput predictions, particularly for competent ores (Bailey et al.,

2009), motivated Ausenco to benchmark the Ausgrind model. This section includes comparisons of theAusgrind outcomes with operational data of the SABC circuit from Newcrest’s Cadia concentrator and

also other published approaches.

Cadia

Ausgrind was used to calculate the energy requirements of the Cadia SABC circuit using information

available in the literature (Dunne et al., 1999, Dunne et al., 2001, Hart et al., 2001). The grinding circuit

data input used for the calculation are listed in Table 3. The information on ore characteristics in this table

refers to the monzonite ore. The SAG mill operating conditions were obtained from Dunne et al. (2001).

Table 3 Cadia grinding circuit data input for Ausgrind calculations

Ore Value Unit SAG Mill Data Value Unit

JK or SMC parameters (A×b) 36 No Mills 1

SG 2.7 t/m3 Diameter (inside shell) 12.2 m

DWi 7.5 Aspect ratio (L/D) 0.5

Bond crusher work index 16.5 kWh/t Effective Grinding Length, EGL 6.1 m

Bond rod mill work index 19.0 kWh/t Fraction of crit. speed 0.72

Bond ball mill work index 17.1 kWh/t Ball volume (nominal design) 12 %

Total filling (nominal design) 25 %

Crusher Circuit Value Unit Discharge slurry % solids 70 %

Throughput - Primary crusher 4000 t/h

Primary crusher circuit F80 335 mm Ball Mill Data Value Unit

Primary crusher circuit CSS 120 mm No Mills 2

Crushing circuit P80 98 mm Diameter 6.7 m

Aspect ratio (L/D) 1.68

Grinding Circuit Value Unit Belly length inside liners 11.3 m

Throughput - Grinding 2065 t/h Fraction of crit. speed 0.72

Cyclone overflow P80 170 micron Ball volume 30 %

Select circuit type SABC Discharge slurry % solids 70 %

The specific energy measured during the survey (Dunne et al., 2001) and that calculated using Ausgrind



are listed in Table 4.

Table 4 Ausgrind calculations for the Cadia SABC circuit

MethodSpecific energy - Ecs (kWh/t) ∆

(kWh/t)SAG mill Ball mill Total

Ausgrind 8.8 8.8 17.6 -

Survey (1998) 8.6 8.0 16.6 - 1.0

Comparison with Other Methodologies

The Ausgrind method was compared to other techniques described in this paper using values published

in journal and conference papers available in the public domain. Seven papers were selected: Morrell,

2009; Starkey and Holmes, 2001; Marks et al., 2001; Becerra and Amelunxen, 2012; Burgess, 2012 andDelboni et al., 2006. The results are presented in Table 5 and contain comparative calculations for the

following cases:

Morrell’s (2009) paper describes the SMC methodology for predicting the overall Ecs of comminution

circuits. The information regarding the SABC worked example was used for calculating and

comparing Ecs.

Starkey and Holmes’ (2001) paper shows the design of the Kubaka gold ore grinding circuit using SPI

and Bond test parameters. According to the authors, the previous characterization testwork was

carried out for the harder ores of the deposit and included the MacPherson autogenous work index,

Bond rod and ball mill work indices and SPI tests. Both MacPherson and Starkey power estimates are

compared to the Ausgrind calculation.

The Similco SACB grinding circuit was designed using the JKTech and SGS characterization databases

which included JKDWT and Bond rod and ball mill work indices (Marks et al., 2001). For design of the

circuit, the authors used the FLSmidth in-house mill selection criteria.

Becerra and Amelunxen (2012) published the study for the Confluencia SACB grinding circuit at the

Los Bronces copper deposit. In this study, three methodologies were used to analyze the energy

requirements: MillPower, JKSimMet and Aminpro in-house methodology. The comminution testwork

conducted included Bond rod and ball mill work indices, Bond abrasion index, SPI and SMC testes.

Median values for all the parameters were used for comparison with the Ausgrind calculations.

The DBC method for calculating AG/SAG Ecs uses a combination of Bond work indices and the drop

weight test parameters. The author published a paper (Burgess, 2012) comparing his methodology

with the SMC method for three ore competencies (soft, medium and hard) and producing a final

product of 0.075 mm, 0.107 mm and 0.75 mm, respectively. The single stage SAG circuit was used for

the selected three examples. Since Ausgrind does not consider this circuit configuration for hard ores,

only Ecs values for soft and medium ores were calculated for comparison.

The Sossego SABC grinding circuit was dimensioned under the consultancy of Delboni (2006) using a

combined method of JKSimMet and an in-house power-based empirical model.



Table 5 Ausgrind benchmarked against other methodologies

ExampleCircuit

configurationMethod

Specific energy (kWh/t) ∆

(kWh/t)SAG mill Ball mill Total

SMC worked

exampleSABC

Ausgrind 8.1 10.0 18.1 -

SMC 9.6 8.4 18.3 + 0.2

Kubaca circuit SAB

Ausgrind 11.8 14.1 25.9 -

MacPherson 10.9 14.0 24.9 - 1.0

Starkey 13.4 16.4 29.8 + 4.1

Similco circuit SACBAusgrind 12.0 14.6 26.6 -

FLSmidth 8.0 16.0 24.3 - 2.3

Confluencia

circuit

SACB

Ausgrind 5.7 10.4 16.1 -

MillPower 6.2 9.2 15.7 - 0.4

JKSimMet 5.5 9.5 15.3 - 0.8Aminpro 6.3 8.3 14.9 - 1.2

DBC worked

example - soft

ore

SSAG

Ausgrind 16.6 - 16.6 -

DBC 17.4 - 17.4 + 0.8

SMC 16.8 - 16.8 + 0.2

DBC worked

example -

medium ore

SSAG

Ausgrind 18.0 - 18.0 -

DBC 17.0 - 17.0 - 1.0

SMC 17.2 - 17.2 - 0.8

Sossego circuit SABCAusgrind 9.6 6.7 16.3 -

Delboni 10.8 8.7 19.5 + 3.2

The delta ( ∆ ) in Table 5 is the difference in the total grinding specific energy between calculations with

Ausgrind and the other methods. According to Table 5, the lowest delta values were obtained when

comparing Ausgrind and the SMC method (+ 0.2 and – 0.8 kWh/t). The highest delta values were obtainedwhen comparing Ausgrind and Starkey’s method for the Kubaca circuit (+ 4.1 kW/t, a 15% difference) and

Delboni’s method for the Sossego circuit (+ 3.2 kWh/t, a 20% difference). The specific energy predictions

for the other examples resulted in delta values lower than 2.3 kWh/t, meaning that the relative difference

was within a range from 2 to 9%.

Comparison with Plant Surveys

Ausgrind specific energy predictions were compared to measured values during surveys in three

comminution circuits which processed hard ores. The results showed good agreement, and are presented

in Figure 6.



Figure 6 Ausgrind benchmark against plant surveys

CONCLUSIONS

A number of well-established methodologies used for calculating comminution energy requirements have

been presented in this paper. The published results from these methodologies were used to benchmark

the Ausgrind program. Ausgrind reproduced the specific energy requirements calculated by other

methods for various projects within ± 10% accuracy, with only a few exceptions, and predicted within 5%

the survey data from four different operations. Therefore, Ausgrind has demonstrated that its

methodology is a suitable and reliable method for designing AG/SAG milling circuits.

Every approach discussed in this paper relies on the use of efficiency factors (directly or indirectly) to

correct the Bond comminution circuit energy requirements for an AG/SAG circuit. Some methods are

more conservative than others, resulting in different efficiency factors and estimated energy requirements.

Notwithstanding the merits of comminution methodologies, the representativity of sample selection, the

applicability of the testwork and the quality of testwork data are critical for the success of any circuit

design. In light of this, the Ausgrind method includes a guideline for best practice in obtaining quality

input data.

ACKNOLEDGMENTS

The authors would like to thank Ausenco Services for the permission to publish this paper. Eddie McLean

is also acknowledged for his valuable comments and revision on the paper.

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Power-based Comminution Calculations Using Ausgrind